This invention relates to improvements in buoyant structures of the type particularly useful in undersea environments and improvements in their manufacture.
U.S. Pat. No. 3,622,437 to Sidney D. Cook, the disclosure of which is incorporated herein by reference, discloses a buoyancy material which exemplifies the type of structure with which the present invention is concerned. The structure disclosed therein comprises a plurality of generally spherical, low density, buoyant bodies encased in a matrix of a lightweight material known as syntactic foam, i.e., a hardenable resin loaded with hollow microspheres which serve to lower its density.
Such buoyant materials have found many uses as the sea has increasingly been utilized as a resource. For example, they are used to impart buoyancy to submergible equipment such as deep sea research instruments, cables, pipelines, and marine riser pipe such as are used in off-shore oil drilling operations. Their commercial success may be traced primarily to the fact that they provide a relatively inexpensive buoyancy material which is capable of withstanding relatively high hydrostatic pressures. Furthermore, unlike single-walled hollow pressure vessels which are subject to catastrophic failure when the wall is penetrated, the buoyancy materials with which the instant invention is concerned comprise a compartmentalized structure which tends to localize any failure due to implosion and thereby to retain most of its buoyancy.
It is well known that strength and impact resistance of buoyant materials of the type described may be improved by applying a protective skin, e.g., a fiberglass resin laminate. However, this type of skin, which must be excessively thick in order to provide suitable protection to the buoyant bodies, is characterized by a substantially increased weight.
U.S. Pat. No. 4,021,589 to Copley discloses a buoyancy material having a layered integral skin which comprises an inner layer of syntactic foam of a thickness sufficient to prevent implosion of buoyant bodies located in the outer portions of the buoyancy material when subjected to a desired hydrostatic pressure, and an outer layer of resin permeated reinforcing material to add strength and protect the buoyant core and the inner layer against impact damage. The outer layer of the skin also aids in preventing implosion of the buoyant bodies at desired hydrostatic pressure, but, due to the fact that it is much thinner than the inner layer of the skin, its effect in this regard is minor. The inner layer may contain a spacer means imbedded in the resin which functions to retain the buoyant bodies inwardly from the surface of the finally formed article during its manufacture. The spacer means comprises a composition of enmeshed filamentary material, e.g., fibers of plastic, such as is manufactured by American Enka Corp. and sold under the trademark ENKAMAT.RTM.. The skin, including the spacer means, is coextensive with the buoyancy material, and thus deleteriously increases the weight of the buoyancy module.
The aforementioned buoyant materials have previously been manufactured by packing hollow or foam filled, thin walled, generally spherical buoyant bodies ranging, in general, from one-sixteenth inch to about 6 inches in diameter, in a mold of a desired shape, and thereafter filling through a plurality of fill holes the interstices among the buoyant bodies with a syntactic foam. The syntactic foam serves as a matrix to encase the implodable buoyant bodies and reinforces the walls thereof. However, the use of multiple fill holes as well as other factors involved in the manufacture of the buoyant bodies has resulted in the presence of air voids in the final product. In addition, the implodable buoyant bodies, which are a relatively bulky filler, tend to impede the flow of the resin, which results in the discontinuous distribution of the syntactic foam filler. A plurality of fill holes can be used in an attempt to evenly distribute the resin, but this can add to the air-entrapment problem. Moreover, the use of a plurality of fill holes does not completely eliminate the discontinuous distribution of the resin, and it leads to excessive time and labor associated with connecting and disconnecting the numerous fill holes from the foam supply container.